Recently, HCCI combustion in which gasoline fuel mixed with air is combusted by self-ignition inside a sufficiently compressed cylinder has attracted attention. This HCCI combustion has an issue that an ignition timing of mixture gas (a timing that the mixture gas self-ignites) greatly varies due to an external factor (e.g., atmospheric temperature) and an issue that a control during a transient operation in which an engine load sharply changes is difficult.
Therefore, instead of combusting all the mixture gas by self-ignition, it is proposed to combust a portion of the mixture gas by spark-ignition using a spark plug. That is, after forcibly combusting a portion of the mixture gas through flame propagation caused by spark-ignition (SI combustion), the rest of mixture gas is combusted by self-ignition (CI combustion). Hereinafter, such combustion is referred to as “partial compression-ignition combustion.”
By forcibly combusting the portion of the mixture gas through flame propagation, the subsequent CI combustion is easily achieved. However, even in this case, the ignition timing of the CI combustion fluctuates, which leads to combustion noise fluctuation, due to the external factor, and the combustion noise may become excessive in the CI combustion.
JP5839972B discloses one method of controlling a combustion state of mixture gas, in which a sensor which detects pressure inside a cylinder (in-cylinder pressure) is provided, a combustion center-of-gravity timing (a timing at which 50% of a mass of the fuel supply into the cylinder combusts) based on the in-cylinder pressure detected by the sensor is calculated, and an ignition timing, etc. are suitably controlled so that the combustion center-of-gravity timing becomes suitable.
However, in the partial compression-ignition combustion, the SI combustion and the CI combustion occur as described above. The combustion center-of-gravity timing also changes depending on a ratio between the SI combustion and CI combustion, etc. Therefore, in the partial compression-ignition combustion, simply controlling the combustion center-of-gravity timing to be suitable may not sufficiently reduce the combustion noise caused by the CI combustion.